In European Patent Application No. 95830371.1-2207, filed on Aug. 31, 1995, and assigned to the present assignee, there is described a PWM driving system for an inductive actuator whose block diagram is shown in FIG. 1. The system comprises a control loop for the current flowing in an inductive load (R.sub.L -L.sub.L) with the power stage functioning in a PWM mode at a constant frequency.
The distinguishing feature of the illustrated system is that the current through the load is controlled by varying the duty-cycle of the two outputs (OUT+ and OUT-) of the POWER AMPLIFIER stage, and, more precisely, the condition of null current is obtained by driving the two outputs with two signals having the same frequency and a 50% duty cycle. By increasing the duty cycle of the output OUT+ and decreasing the duty cycle of the other output OUT-, or vice versa, the current through the load will circulate with a direction and intensity dependent on the difference of duty cycle difference between the two outputs.
The current sensing in this example is implemented by way of a sensing resistor Rs in series with the load. The voltage generated on this resistor is amplified by an operational amplifier SENSE AMPL. and is used as feedback input by the ERROR AMPL. to close the control loop. A critical point of this system is indeed the sensing of the current, and, more precisely, the generation of a voltage signal proportional to the mean value of the current that flows through the load. In fact, in this prior art system this is simply obtained through an operational amplifier with inputs fed a differential voltage on the order of hundred of millivolts. The voltage is represented by the voltage drop caused by the load current on the sensing resistor and a common mode voltage that goes from ground potential to the supply voltage Vp because the power stage functions in a PWM mode.
FIG. 2 shows the waveforms of the system described in the above cited patent application which show the relationships between the load current, the output voltage of the SENSE AMPL. and the two outputs OUT+ and OUT- of the power stage. It may be noticed that the output voltage of the SENSE AMPL. is perturbed by the switching of the two outputs of the power stage. Wishing to control the mean value of the current through the load, the gain and the passband of the current control loop must be limited in a manner to filter the noise existing on the output voltage of the SENSE AMPL. representing the feedback signal.